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instance vessels of greater capacity, say 51., an experimentation 

 time of 5 hours and ^ solutions. 



Producing different intensities of light. When studying the 

 influence of light on G0 2 assimilation one should be able to vary 

 the intensity of light. Variations were obtained by covering the 

 receiver with the leaf with screens of different transparency. Paper 

 screens that were used at first are not suitable because they do not 

 absorb the different rays in the same degree. The best screens are 

 prepared from photographic plates, faintly illuminated and develop- 

 ed in the usual manner. By preparing a series of such screens and 

 combining them, different intensities of light can be produced. 



Although estimations in subdued light are carried out with 

 facility, it is rather difficult to determine the G0 2 assimilation in 

 direct sunshine. The temperature in the plant receiver easily rises 

 too high and the leaf is injured. Yet the determinations can be car- 

 ried out with the following precautions. The receiver is fitted in a 

 stand, 30 cm above the table. The sunlight is filtered through a 

 shallow disc with water, applied some cm above the receiver. It is 

 still better to use the arrangement in my experiments with Sinapis: 

 as receiver to use a glass tube surrounded with another glass tube 

 with side tubes, through which a water current passes during the 

 experiment. 



2. Estimation of the intensity of light. When experiments 

 on the relation between G0 2 assimilation and the intensity of light 

 are to be carried out, it is necessary to make determinations of the 

 intensity of illumination. Wiesner (1907) who especially has in- 

 vestigated this question, used silverchloride paper after the method 

 of Bunsen-Roscoe; but this paper loses its sensibility very quickly 

 and must be prepared anew every day; therefore both Wiesner 

 and other investigators have, besides the silverchloride paper, used 

 photographic papers of different kinds. 



When I studied the influence of light upon forest-trees, I used 

 Wynnes photographic exposure-meter, which is also used by several 

 others (Rübel 1908 p. 13.) 



Estimations with this apparatus are carried out in this 

 manner, that the time, in which a sensitive paper is coloured to 

 the same degree as the standard tints on the meter, is measured. 

 This time is inversely proportional to the intensity of illumination. 

 When the paper is coloured in 5 minutes, the intensity of illu- 

 mination is ^ ; in order to obtain convenient values this figure may 



